TOF MS is a fast, efficient and inexpensive technique for discerning the mass of macromolecules. One prominent example for TOF MS is MALDI TOF MS. For MALDI analysis the sample molecules are mixed with a light-absorbing matrix and are vaporized and ionized using a short laser pulse. The molecular ions are then accelerated by a high voltage (+/−10 to 30 kV) through an evacuated tube of a known length and their arrival times at the opposite end are recorded. Measuring the flight time of the molecular ions between the laser pulse (start signal) and the detector signal (stop signal) allows one to calculate the mass to charge ratio of the ions. Because in MALDI the charge of the ions is typically +1, the mass is easily discerned.
Conventional mass spectrometers for biomolecular analysis typically use microchannel plates (MCP) to measure the arrival times of the molecular ions. An ion impacting onto the front surface of the MCP can produce secondary electrons which are then multiplied within the MCP and an output signal as a burst of electrons is measured as current. For large molecules, i.e. exceeding 10,000 atomic mass units, the velocity attained during a typical TOF experiment is typically too low to produce secondary electrons efficiently when impacting on the surface of the MCP. Thus, the detection efficiency of an MCP drops dramatically for large masses in existing TOF MS systems when relying directly upon ion-to-electron conversion, as demonstrated by J. Martens, W. Ens and K. G. Standing in “Proceedings of the ASMS 1991” with a mass of 66,000 u. For example, an organic molecule ion, mostly carbon and hydrogen, with a mass of 50,000 atomic mass units is made up of approximately 5,000 atoms. Even using an acceleration voltage up to 30 kilovolts (the current practical limit), and assuming singly charged particles, as is common in MALDI, only approximately 6 electron volts of kinetic energy is carried on average by each atom. For larger ions in the mass range from 100,000 to 1,000,000 atomic mass units, the energies are even lower and ion detection, therefore, much more difficult.
The generation of secondary electrons at a surface is essentially dependent on the velocity of the impinging ions. The heavy ions fly very slowly and are hardly able to release any secondary electrons upon impact (R. J. Beuhler and L. Friedman, “Threshold Studies of Secondary Electron Emission Induced by Macro-Ion Impact on Solid Surfaces”, Nuclear Instr. Methods, 170 (1980) 309-315 309; A. Brunelle, P. Chaurand, S. Della-Negra, Y. Le Beyec and E. Parilis; A. Brunellea, P. Chauranda, S. Della-Negra, Y. Le Beyeca and G. B. Baptista, “Surface secondary electron and secondary ion emission induced by large molecular ion impacts” Int. J. Mass Spectr. Ion Proc. 126 (1993) 65-13;). Large molecules more readily generate secondary ions by a sputtering process, rather than releasing secondary electrons. The utility of existing MALDI TOF MS for studying large biomolecules is therefore severely limited by the lack of detector sensitivity at high masses.
Additionally, because of the basic design of a TOF experiment, lighter mass ions impact the detector first, followed later in time by heavier ions. MCPs are made of an array of tubes or channels, i.e. microchannels, which multiply electrons as they pass through them. Each tube can be considered as an individual dynode with its own dynode resistance on the order of approximately 1014 Ohm (J. L. Wiza, “Microchannel Plate Detectors”, Nuclear Instruments Methods 162 (1979) 587-601). The recovery time for these tubes once discharged is on the order of tens of milliseconds, which is several orders of magnitude longer than the duration of the high mass ions during TOF experiment, which is hundreds of microseconds. (S. Coeck, M. Beck, B. Delaure, V. V. Golovko, M. Herbane, A. Lindroth, S. Kopecky, V. Yu. Kozlov, I. S. Kraev, T. Phalet, N. Severijns, “Microchannel plate response to high-intensity ion bunches”, Nuclear Instruments Methods in Physics Res. A, 557 (2006) 516-522). Therefore, once smaller mass ions deplete the charge of an individual channel, that channel is saturated (turned off) for the remainder of the TOF experiment. This saturation effect causes an additional sensitivity bias making it increasingly more difficult to measure high mass ions. This becomes especially problematic in complex sample mixtures such as biologic or polymer samples; however, even relatively pure samples routinely contain multiple signals (i.e. matrix, multimers, multiple charges, adducts) which can cause saturation bias.
Thus, there is need for improving the sensitivity and saturation problems of ion detectors to improve the mass range accessible by MALDI TOF MS.
One method to increase sensitivity for high mass ions is to add a conversion dynode, onto which the ions impact, for use in combination with a standard detector, i.e. MCP, typically used for ions of smaller masses. The conversion dynode can be designed of any surface as in the U.S. Pat. No. 5,202,561 (Giessmann, Hillenkamp, Karas), a flat plate as in DE U.S. Pat. No. 4,129,791 (Holle, A), an MCP as in U.S. Pat. No. 6,051,831 or as a “Venetian blind” as in U.S. Pat. No. 5,463,218 (Holle). This Venetian blind consists of a flat device perpendicular to the ions flight direction made up of a multiple rows of metal stripes, each rotated to approximately 45° to the flight direction, thus creating an impassable barrier for the ions. Behind the Venetian blind, there is an accelerating field which draws out the resulting secondary ions from the Venetian blind and accelerates them toward the ion detector.
These secondary ions which are produced from the Conversion Dynode surface vary in mass typically from 1 to 200 mass units. They must be then reaccelerated and undergo a second minor “time-of-flight” dispersion before impacting a second surface where they can be detected, often by conversion to electrons, which are amplified and finally detected as current through a load resistor. This second “time-of-flight” causes a spread in the impact time relative to the original ion packet because of the differing flight times between the different secondary ion masses.
In U.S. Pat. No. 5,463,218 (Holle) a conversion dynode and a MCP are arranged at a very short distance of a few millimeters from each other in order to minimize for such a time spread. However, the conversion dynode is at ground potential and the potential difference between dynode and MCP, and therefore the acceleration of the secondary ions, is strongly limited due to limited insulation properties. In addition, a scintillator plate is inserted after the MCP, in part to convert electrons into photons to be detected and in another part to insure electrical insulation for the high voltage between the scintillator front end and the detection side. Following the scintillator isolation/conversion process, the photons are detected by a photomultiplier detector to minimize or eliminate saturation of the final signal.
In U.S. Pat. No. 5,202,561 (Giessmann, Hillenkamp and Karas), a method is described by which after impacting the conversion dynode the small secondary ions back away from the conversion dynode, which transfers a more or less uniform energy to them. In addition, there can be a magnetic cross field in front of the conversion dynode which forces the extracted ions onto a circular path which allows them to impact on a multichannel array detector after a 180° deflection for further amplification via secondary electrons. Here a slit is arranged which filters out ions of undesirable masses and allows only the ions of a specific mass to continue flying providing relatively equal flight time for the converted ions. The particles transmitted from the conversion dynode can also be accepted by a secondary electron multiplier arranged in the direction of radiation. However, this rather complicated arrangement including the separation and filtering out of secondary ions drastically limits the sensitivity.
It has been found that the use of conversion dynodes (CD) followed by secondary electron multipliers (SEM) solves some of the sensitivity problems of the TOF MS detection because the CD-SEM detector does not rely on direct ion-to-electron production. However, lack of sensitivity and saturation problems still exist, especially for high mass ion detection.
In another aspect of TOF MS different requirements demand for different detectors. However, to measure ions using different detectors it is necessary to break the vacuum of the mass spectrometer system and physically change the detector. Because the vacuum needed to operate these detectors and mass spectrometers is typically 10^-6 mbar or lower and some systems have to be baked out before reuse, it often takes hours or longer for the mass spectrometer to pump down to these pressures after reaching atmospheric pressure. During this process of changing detectors often the sample will deteriorate making it very difficult to monitor the same sample with different detectors.
Because each detector design exhibits its own advantages and disadvantages, it should be useful to have a device to easily switch between detectors.